There are two questions you need to ask:
1) You need to ask your Building Standards department what their position is on electrical work not carried out by an approved CoC.
2) You need to ask yourself if, and why, you genuinely believe that you could sign a declaration which reads
I being the person responsible for the Design, Construction, Inspection & Testing of the electrical installation (as indicated by my signature below), particulars of which are described above, having exercised reasonable skill and care when carrying out the Design, Construction, Inspection & Testing, hereby CERTIFY that the said work for which I have been responsible is to the best of my knowledge and belief in accordance with BS 7671:2008 amended to 2011 except for the departures, if any, detailed as follows:
And, although I am not saying that you could ever be in that position, your degree of certainty should be high enough that you would be happy to defend it under oath in court. In other words don't DIY if your belief is "well I think I could do it OK" - you must be damned sure, and know why you are damned sure, that it will be OK.